The Mets’ game against the Diamondbacks on Friday was postponed due to rain, and will be made up Sunday as part of a single-admission doubleheader.

Rookie Rafael Montero will start the first game, at 1:10 p.m., while Daisuke Matsuzaka will start the second game, slated to begin a half-hour after the first ends, on a 100-pitch count. Josh Collmenter will start Saturday for the Diamondbacks, and Bronson Arroyo will get one of the two Sunday starts.

“I’d have no problem with 100 pitches on Sunday,’’ pitching coach Dan Warthen said of Matsuzaka, who pitched in relief Thursday, but doesn’t anticipate having a problem going back into a starter’s role.

“I pitched [Thursday] but it was just one inning. I didn’t throw too many pitches so throwing 100 pitches shouldn’t be a problem,’’ Matsuzaka said through a translator. “It’s been a while. It’s something I’ve been doing, I know how to do. I don’t want to think too deeply into it; I just want to go into the game and do my role.’’

Matsuzaka, whose season high in pitches was 56 in 3 ²/₃ innings on May 13, said he wasn’t viewing this as an opportunity to prove anything to Terry Collins and work his way back into the starting rotation.

“No, I don’t see it being anything long term; it’s probably just for Sunday,’’ Matsuzaka said. “But Sunday I will be starting the game, so I have a responsibility as a starter to pitch deep into the game, and that’s what I expect to do.’’

Warthen said the Mets will have a 26th man available, because MLB allows an extra roster spot for a doubleheader, although he added no decision had been made as to who that would be.

Tickets for Sunday’s regularly scheduled game (the first one) are valid for admission, while those holding a paid ticket for Friday’s game can exchange it for a ticket of comparable price and location within the next 12 months.

Travis d’Arnaud ran, took batting practice outdoors and was expected to catch a bullpen session Friday as he continues his comeback from a concussion.

“If he’s cleared by the trainers and cleared by the doctors that see him on a daily basis here, the next step is to get major league baseball to say, ‘OK, he’s ready to start baseball activities,’ ’’ Collins said before the Mets were rained out. “They review our records and sign off and put him on the field.

Collins said he’d still expect d’Arnaud to need a rehab assignment and get at least eight-to-10 at-bats and catch at least seven innings, but didn’t have an idea when that might happen.

With two of d’Arnaud’s three concussions coming wearing his hockey-style mask, he said he’ll go back to his old mask.

Jose Valverde had been expected to handle the closing duties Friday if needed, with Jenrry Mejia unavailable after pitching back-to-back days for the first time since 2010.

But Mejia came through OK, clearing an important hurdle. What’s the last step to be named the full-time closer?

“He’s got to go out there three or four times in a row, I can tell you that, because that’s what you’re going to count on,” Collins said. “If you’re going to win at this level, you’ve got to have that guy you can count on down there. I don’t think it’ll be very long three nights in a row is [no longer] out of reach.”

Since moving to the bullpen, the 24-year-old Mejia hasn’t allowed a run in 5 ¹/₃ innings, striking out six without a walk.

“I feel good. Everything is great,’’ said Mejia, initially hesitant about moving to the bullpen for fear of re-injuring his arm, but now hopeful of earning the closer role. “I feel happy about it. I work hard to be here and if they give me that spot I’m going to be happy and feel comfortable because I’m going to know when I’m going to pitch and know my innings, so that’s going to make me feel happy.’’

Dillon Gee, out since straining a lat muscle on May 10, may miss more than three turns through the rotation. He wanted to make a rehab start Sunday or Monday, but since he’s still just throwing off flat ground and didn’t throw off the mound Friday, that seems unlikely.

Collins, though, said Gee continues to improve.

Wilmer Flores is hitting .364 over his last six games, but just as importantly, he’s made all the plays at shortstop he’s supposed to.

“I was waiting for somebody to say, ‘Do you think Ruben [ Tejada] would’ve caught that?’ That hasn’t been said yet — nobody’s even brought it up,” Collins said. “That tells me he’s done exactly what we hoped he’d do: Catch the balls he’s supposed to catch. That’s going to be the key. Do we expect him to be Ozzie Smith? No. But there’s no reason he can’t be that guy who is an offensive minded shortstop.’’